The process industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gases in chemical, pulp, petroleum, pharmaceutical, food and other fluid process plants. Process variables includes pressure, temperature, flow, level, turbidity, density, concentration, chemical composition, and other properties.
A process fluid temperature transmitter provides and output related to a process fluid temperature. The temperature transmitter output can be communicated over a process control loop to a control room, or the output can be communicated to another process device such that the process can be monitored and controlled.
Traditionally, process fluid temperature transmitters were coupled to or employed thermowells which provided a temperature sensor in thermal communication with a process fluid but otherwise protected the temperature sensor from direct contact with the process fluid. The thermowell is positioned within the process fluid in order to ensure substantial thermal contact between the process fluid and the temperature sensor disposed inside the thermowell. Thermowells are typically designed using relatively robust metal structures such that the thermowell can withstand a number of challenges provided by the process fluid. Such challenges can include physical challenges, such as process fluid flowing past the thermowell at a relatively high rate; thermal challenges, such as extremely high temperature; pressure challenges, such as the process fluid being conveyed or stored at a high pressure; and chemical challenges, such as those provided by a caustic process fluid. Further, thermowells can be difficult to design into a process installation. Such thermowells require a process intrusion where the thermowell is mounted to and extends into a process vessel such as a tank or pipe. This process intrusion itself must be carefully designed and controlled such that the process fluid does not leak from the vessel at the intrusion point.
There are a number of factors that can compromise the structural integrity of a thermowell. In some cases, not all factors may be fully considered and thermowells have sometimes bent or even broken off thus causing the process installation to be shut down for a significant period of time. This is highly undesirable. For some applications, a thermowell simply cannot be used without potential damage. In such applications, it may be beneficial, or even required, to use a non-invasive process fluid temperature calculation system. With such a system, a pipe clamp sensor is used to couple a temperature sensor to a process vessel, such as a pipe. While such a non-invasive process fluid temperature calculation provides the benefit of not requiring a process intrusion, nor subjecting a thermowell directly to the process fluid, there is a tradeoff. Specifically, a non-invasive temperature calculation system is typically less accurate in detecting the process fluid temperature than a thermowell which extends into the process fluid and measures the temperature directly.
Providing a non-invasive process fluid temperature calculation system that could more accurately reflect the temperature of the process fluid would reduce some of the tradeoff required by users of such systems and also potentially provide more accurate temperature calculation and process control in situations where thermowells were not desired or possible.